#include <iostream>
#include <stack>
#include <vector>
using namespace std;

struct TreeNode
{
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode() : val(0), left(nullptr), right(nullptr) {}
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
};

class Solution
{
public:
    vector<int> preorderTraversal(TreeNode *root)
    {
        stack<TreeNode *> st;
        TreeNode *cur = root;
        vector<int> v;
        while (cur || !st.empty())
        {
            while (cur)
            {
                v.push_back(cur->val);
                st.push(cur);

                cur = cur->left;
            }
            TreeNode *top = st.top();
            st.pop();
            cur = top->right;
        }
        return v;
    }
};